CAT661 データシートの表示（PDF） - ON Semiconductor
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CAT661 Datasheet PDF : 16 Pages
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CAT661
Application Information
Circuit Description and Operating Theory
The CAT661 switches capacitors to invert or double an
input voltage.
Figure 19 shows a simple switch capacitor circuit. In
position 1 capacitor C1 is charged to voltage V1. The total
charge on C1 is Q1 = C1V1. When the switch moves to
position 2, the input capacitor C1 is discharged to voltage
V2. After discharge, the charge on C1 is Q2 = C1V2.
The charge transferred is:
DQ + Q1 * Q2 + C1 (V1 * V2)
If the switch is cycled “F” times per second, the current
(charge transfer per unit time) is:
I + F DQ + F C1 (V1 * V2)
Rearranging in terms of impedance:
I
+
(V1 * V2)
(1ńFC1)
+
V1 * V2
REQ
The 1/FC1 term can be modeled as an equivalent
impedance REQ. A simple equivalent circuit is shown in
V1
V2
Figure 20. This circuit does not include the switch resistance
nor does it include output voltage ripple. It does allow one
to understand the switch−capacitor topology and make
prudent engineering tradeoffs.
For example, power conversion efficiency is set by the
output impedance, which consists of REQ and switch
resistance. As switching frequency is decreased, REQ, the
1/FC1 term, will dominate the output impedance, causing
higher voltage losses and decreased efficiency. As the
frequency is increased quiescent current increases. At high
frequency this current becomes significant and the power
efficiency degrades.
The oscillator is designed to operate where voltage losses
are a minimum. With external 150 mF capacitors, the internal
switch resistances and the Equivalent Series Resistance
(ESR) of the external capacitors determine the effective
output impedance.
A block diagram of the CAT661 is shown in Figure 21.
REQ
V1
V2
C2
C1
RL
C2
RL
REQ
+
1
FC1
Figure 19. Switched−Capacitor Building Block
Figure 20. Switched−Capacitor Equivalent Circuit
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